In processes where nickel or cobalt is electrowon from electrolytes containing sulfate and/or chloride ions, the metal in question is deposited at the cathodes while, depending on the anions present, chlorine or oxygen is evolved at the anodes. The evolution of oxygen in the case of sulfate electrolytes is accompanied by a lowering of the pH due to sulfuric acid formation.
A permeable membrane or diaphragm is placed between each anode and cathodes adjacent to it. In the case of electrowinning from chloride electrolytes the diaphragm serves the purpose of confining the anodically generated chlorine to the vicinity of the anodes, which can be provided with suitable hoods for avoiding environmentally objectionable fuming. In the case of electrowinning from sulfate electrolytes the diaphragm serves to confine the low pH conditions to the vicinity of the anode, thereby ensuring that the bulk of the electrolyte remains at the less acidic pH needed to ensure that nickel or cobalt deposition occurs in preference to hydrogen evolution.
In order to prevent back-diffusion of chlorine or acidic anolyte into the bulk electrolyte compartment, a positive flow of electrolyte is maintained from cathode to anode through the diaphragm. One way of accomplishing this flow involves using a diaphragm which surrounds an anode, i.e., a diaphragm bag, so that the space within each anode bag constitutes an anolyte compartment, while the space outside the bags constitutes the bulk electrolyte of the cell. Such an arrangement is described for example in co-pending application for letters patent, Ser. No. 288,455, filed on Oct. 17, 1977 and assigned in common with the present invention.
A problem which is encountered in any such bagged anode operation is the difficulty in maintaining identical conditions in the various anolyte compartments of a given tank. Should there be variations between the rate at which electroltye is withdrawn from the various anolyte compartments, the resulting high anolyte level creating an undesirable hydrostatic head in combination with the constantly growing acid concentration can cause back diffusion from one or more of the anode bags and this in turn leads to current inefficiency and possibly environmental problems if acid mist or chlorine gas escape from the unhooded bulk electrolyte.
In the past, it has been suggested to withdraw gases by suction from the sealed space above each anolyte, and to withdraw the anolyte itself from a discharge outlet provided in the side of the anode bag. Alternatively, it has been proposed to remove both liquid and gases from a single outlet in the side of the anode bag. With either of these arrangements, we have found flow equalization difficult to achieve. Relatively small variations, such as a fraction of an inch, between the elevations of the various anolyte outlets leads to unacceptable variations in the anolyte flow.